One area of refrigeration that has received considerable attention in recent years is cryogenics. For many applications, there is a growing demand for refrigeration systems that are capable of cooling an instrument to cryogenic temperatures (below about 120.degree. Kelvin) in order to obtain desired performance.
A particularly challenging application of refrigeration systems is cooling spacecraft sensors to cryogenic temperatures. Various spacecraft sensors, such as certain electromagnetic or infrared sensors, require cryogenic cooling or can more readily detect small incident signals from the earth or space when they are cooled to cryogenic temperatures. In addition, spacecraft instruments such as semiconductor lasers, whose frequencies are extremely sensitive to temperature, require a refrigeration system that is capable of maintaining such devices at a predetermined cryogenic temperature with highly accurate temperature stability. Maintaining temperature stability of an instrument is often complicated by large variations in heat output or thermal load by the instrument, i.e., the heat which must be removed or dissipated to maintain the instrument within the desired temperature range. Spacecraft instruments often exhibit large variations in thermal load due to their operational environment, such as being periodically cycled off/on and the varying incident angle and intensity of solar or earth radiation.
The cryogenic cooling system also must avoid interfering with operation of the spacecraft instruments which typically are highly sensitive to movement or fouling due to discharges, outgassing or the like. For example, vibrations or other movements can exceed sighting tolerances for pointing at a position on the earth, a distant body, or another spacecraft. Moreover, cryogenic cooling system design for space environments is complicated by the fact that the cryogenic cooling system normally undergoes significant gravitation variations in terms or strength or orientation due to factors such as movement relative to the earth or other bodies and spacecraft attitude variations. Spacecraft cryogenic cooling systems must also be compact to fit within a designated spacecraft area and have a mass that is within mission requirements, i.e., that does not unduly increase propellant requirements for launching and maneuvering or decrease payload capacity. Additionally, for many space applications, the cryogenic cooling system must operate for extended periods without human physical intervention.
One known method of cooling an instrument to cryogenic temperatures which has been employed in the space environment involves mounting the instrument on a conductive lead that is cooled by a cryogenic cooling source. The lead may include a rigid mount or a strap which is normally flexible but can become somewhat rigid under certain circumstances. Heat generated by the instrument is conducted through the lead to the cooling source. Due to cooling losses to the environment along the lead, the instrument is typically mounted in close proximity to the cooling source. However, the close proximity makes the instrument susceptible to electromagnetic interference and vibration effects created by the cooling source. For many applications, such as various satellite sensors, the effects of the cooling source can unacceptably interfere with the performance of the instrument. Additionally, for many applications, the conductive lead undergoes significant thermal contraction and expansion as it is cycled between ambient temperatures and cryogenic temperatures, resulting in a corresponding movement of the instrument which can lead to problems such as misalignment and physical stresses or fatigue on the instrument and related structure.
Another known method of cooling an instrument to cryogenic temperatures involves an open-loop cryogenic cooling system. Open-loop cooling systems have an inventory of cryogen which is used to cool the instrument to cryogenic temperatures. In some open-loop systems, the cooling effect of the cryogen is enhanced by allowing the cryogen to boil-off or vaporize thereby taking advantage of thermal absorption properties associated with the phase change. The cryogen is typically exhausted to the ambient environment after absorbing heat from the instrument. A limitation of such open loop systems is that the instrument can be maintained at cryogenic temperatures only during the time in which a supply of cryogen remains. It can be appreciated that in many applications, such as satellite sensors, it is not practical to replenish the inventory of cryogen. Additionally, acceptable cooling system lifespans for many satellite applications would require an unacceptable initial mass of cryogen inventory. Thus, due to the limited operational time of certain open-loop cryogenic cooling systems, these systems are impractical for many space-related and other applications.
A proposed alternative to the open-loop cooling system is a closed-loop cooling system wherein the cryogen is recovered by the cooling system for re-use. Such closed-loop systems have the theoretical advantage of extended useful life because the cryogen is not rapidly exhausted. However, significant obstacles remain with respect to fully realizing these theoretical benefits in a cryogenic system which maintains accurate temperature control of an instrument which has a heat output that varies over time, while being reliable and not unduly complicated or massive.
In addition to the foregoing, known cryogenic systems are commonly subject to one or more of the following limitations relative to ground-based or space-based use.
First, certain known cryogenic cooling systems require the use of a high output or inefficient cooling source in order to maintain an instrument at a constant predetermined temperature when the heat output of the instrument varies over time. This is because these systems respond to changes in instrument temperature by changing the cooling output of the cooling source. That is, the cooling source chases the instrument's heat output. It is inefficient and problematic to vary the cooling output of the cooling source in this fashion. For example, in order to maintain a constant temperature, such cooling systems must have a cooling capacity capable of absorbing the maximum heat generated by the instrument under the most demanding scenarios. In this regard, the heat generated by satellite sensors often varies by over an order of magnitude from a maximum heat output to a minimum heat output. The maximum heat output may occur only occasionally and for short periods of time. Since existing cooling systems must be selected to accommodate the instrument's maximum heat output, there is substantial excess cooling capacity when the instrument is generating less than the maximum heat output. Moreover, "chasing" the instrument's heat output normally involves cycling or major adjustments of the cooling source which can produce vibrations, electromagnetic interference, and/or loss of efficiency.
Another common limitation of some known cryogenic cooling systems is that their cooling output is sensitive to variations in gravity. This sensitivity is due to reliance upon gravity to circulate cryogen through the system or to separate or properly locate desired phases of cryogen in the system. Variations in gravitational field, as are common in space related applications, can render such systems dysfunctional or unreliable.
A limitation, specifically related to Stirling and Pulse Tube cooling systems, is their requirement that the cold producing tip (or cold head) be placed in close proximity to the compressor. This requirement severely limits the ability to place the heat rejecting component (the compressor) near the heat sink (space radiator) and/or to isolate the vibrations of the cooler from the instrument.
Another limitation of some known cryogenic cooling systems is their use of valves that have moving parts, e.g., which vary the valve orifice opening, to control the flow of refrigerant. These valves can suffer mechanical problems at cryogenic temperatures due to substantial contraction and expansion of the moving parts as they are cycled between ambient and cryogenic temperatures. Further mechanical problems are created at cryogenic temperatures when the circulating refrigerant forms deposits on the moving parts.
Known cryogenic systems also commonly service only a single instrument. As can be appreciated, satellites and other electronics platforms often include multiple cryogenic instruments. As a result, it is not uncommon to provide multiple cryogenic cooling systems, each adjacent to a corresponding instrument. It would be useful to reduce the amount of equipment dedicated to cooling. In addition, it would be advantageous to allow for separation of the heat producing instruments from the cryogenic cooling source.